The present invention relates generally to passive optical networks (PON) and more particularly to active real time monitoring of such networks, and to detection of a rogue optical-network unit (ONU) in gigabit PON (GPON) environments.
Passive optical networks, and in particular Ethernet PONs (EPONs) are known, and described for example in U.S. Patent Application No. 20020196801 by Haran et al. The debugging of a running/active network in a shared media network environment is difficult. At the same time, the ability to proactively monitor the network and to verify that its behavior is correct is valuable. Isolating transmission errors (or simply “errors”), detecting the cause of an error and providing debugging tools are highly desired features in a network environment.
A major goal in a PON that comprises an optical line terminal (OLT) and a plurality of optical network units (ONUs) is to detect degradation in the network behavior before customer complains, namely before errors are evident on the line. The most critical aspect is fault isolation, i.e. finding a faulty ONU before it harms the performance of other ONUs. The specific fault of the ONU is less important, because the faulty ONU is likely to be replaced by the network operator.
A PON may suffer from one or more of a number of failure modes (malfunctions or problems), either time-related (“temporal”) or laser-power related (“power”), as shown respectively in FIGS. 1A and 1B. FIG. 1A shows potential temporal malfunctions in a PON comprising 3 ONUs X, Y and Z. In FIG. 1A, the transmission pattern includes two collision zones, a zone 102 between ONU X and ONU Y and a zone 104 between ONU Y and ONU Z. Zone 102 represents a case in which either ONU X stopped transmission after its expected stop time, or ONU Y began transmission after its expected start time. Zone 104 represents a case in which either ONU Y stopped transmission before its expected stop time, or ONU Z started transmission before its expected start time. T1 is the time needed to reach the “sync-lock” or “gain” state of the grant to ONU Y (grant Y), this time also referred to herein as “sync-lock time”. T2 is the time needed to reach the end of grant Y and of the “sync-unlock” or “loss” state (also referred to herein as “sync-unlock time”). “Head overlapping” and “Tail overlapping” refer to heads and tails of grant Y and their overlap with, respectively, a previous and a following grant. In effect, these illustrate the temporal malfunctions of early or late burst reception and early or late end of burst, explained in more detail below:
Early burst reception refers to the case in which an ONU turns-on its laser before the expected time. The outcome may be a bit error rate (BER) in the grant to an ONU immediately preceding the suspected ONU.
Late burst reception refers to the opposite of early burst reception, the reasons being similar. The outcome may be a BER detected in the transmission of the suspected ONU.
Early end of burst refers to the case in which an ONU turns off its laser before the expected time. The outcome could be a BER at the end of its grant. The reasons for an early end of burst may be a faulty ONU or bad ONU timing.
Late end of burst refers to the opposite of early end of burst. The reasons are similar. In this case, the outcome may be a BER at the grant start of the next ONU.
FIG. 1B shows the power transmission pattern of several ONUs a, b, c and d and illustrates potential laser power malfunctions in a PON. The figure shows a normal laser power signal (“ONU burst”) 110 for ONUa as well as three possible power level malfunctions: a weak laser signal 112 for ONUb, a strong laser signal 114 for ONUc and an unstable laser signal 116 for ONUd, all explained in more detail below:
Weak laser signal refers to a failure in which the strength of the ONU signal is lower than expected. This can result from an increase in attenuation or degradation in the ONU's laser power.
Strong laser signal refers to a failure in which the strength of the ONU laser signal is higher than expected. This can result from a faulty operation of the ONU's laser power control.
Unstable laser signal refers to the laser power of a specific ONU being unstable and having random patterns.
A fourth power malfunction is defined as “Laser stuck at 1”, which refers to the situation in which an ONU does not turn off its laser. The laser can transmit random data, idles, or “1”s, with the most likely events being idles and data. This malfunction can have a high impact on the network operation. It also has no specific characterization measurement and its existence is deduced from the behavior of the system.
At present, there are no known methods to detect these malfunctions/problems without intrusive access to the fiber infrastructure and without testing a suspected ONU component in a lab. It would therefore be advantageous to have methods and systems for active real time monitoring (diagnostics) of a PON, which provide information on various failure modes. Preferably, this monitoring should be done without placing-any physical equipment at test points of the PON.
Rogue-ONU detection is one of the biggest challenges for carriers in deploying a time division multiplexed-passive optical network (TDM-PON). The challenge results from malfunctioning or malicious ONUs transmitting at different time periods than the ONUS are assigned to transmit. An ONU is supposed to transmit during, and only during, time intervals dynamically allocated to the ONU. This can lead to a degradation of service for properly-functioning ONUs. A major goal in a PON that includes an OLT and a plurality of ONUs is to detect degradation in the network behavior before a customer complains; in other words, before errors are evident on the line. The most critical aspect is fault isolation (i.e. finding a faulty ONU before it harms the performance of other ONUs). The specific fault of the ONU is less important, because the faulty ONU is likely to be replaced by the network operator.
The ITU-T (ITU Telecommunication Standardization Sector) GPON standard lacks several of the Ethernet PON (EPON) features that allow for simpler detection. Such EPON features include:                (1) “8B/10B” line-coding, which is a coding scheme that translates 8-bit data into 10-bit data and prevents long sequences of 1's and 0's; allowing the system to:                    (a) check the DC balance;            (b) check the comma sync-lock and -unlock time; and            (c) check for code errors; and                        (2) a cyclic redundancy check (CRC) for every packet.        
In contrast, in GPON environments, a scrambler is used to transmit the data without any redundancy. CRC exists only for the last frame field of a packet, and packets may spread over several grants (i.e. uplink transmission from an optical network terminal (ONT)). Furthermore, the sync-unlock time is not available in GPON environments.
In the GPON standard, there is no method to detect a rogue ONU in the network. Received signal-strength indication (RSSI) measurement is a powerful tool for identifying ONU transmission power. RSSI measurements can be used to help detect a rogue ONU, but RSSI measurements cannot be the only source of information. Limitations associated with such means of detection include:                (1) inaccurate power-level measurement, which can be up to 3 dB, prohibiting accurate interference identification;        (2) averaged RSSI measurement (taken over ˜50-100 nanosecond interval), which prevents detection of timing errors; and        (3) limited A/D sampling rate, which prohibits having multiple data points within ONU transmission, preventing real-time response to other indicators.        
It would be desirable to have systems and methods for detecting a rogue ONU in GPON environments. It would be further desirable to have such systems configured for simple hardware (HW) implementation, and preferably flexible software (SW) implementation as well.